Materials that have been believed suitable for use in the electrodes of low surge type vacuum circuit interrupters include Cu-based alloys of low melting, high vapor pressure elements such as Bi, Pb, Te, Se and the like, and Ag-WC series alloys and Cu-W series alloys, both being produced by powder metallurgy technique. The former Cu-based alloys exhibit an excellent low surge voltage characteristic at the initial stage before repetition of breakings. But, when an interrupting current is as large as a short-circuited current, Bi, Pb and the like in the Cu matrix oozes out or evaporation occurs so that the alloy thereafter loses its low surge voltage characteristics. Due to oozing of the low melting, high vapor pressure elements, it is inevitable that the dielectric strength and large current breaking capability is remarkably reduced. Hence, various problems occur in putting these alloys to practical application. The latter alloys such as the Ag-WC series alloys and the Cu-W series alloys also exhibit relatively good low surge voltage characteristics. Among them, the Ag-WC series alloys have good low surge voltage characteristics even after breaking a large current such as a short-circuited current. However, this material involves the problem that it can not break a relatively large current and there is a limit in increasing its capability.
The low surge voltage characteristics will be described in further detail. None of the above-mentioned Cu-based alloys, the Ag-WC series alloys and the Cu-WC series alloys can completely satisfy the low surge voltage characteristics. The low surge voltage characteristics are determined by whether or not the material can minimize the chopping current value at the time of breaking a small current in a vacuum circuit breaker. It is preferred that the material can make the value zero ampere. Practically, however, it is not possible to make it zero. In this sense, the value of about 1 to about 3A has been conventionally used to represent the low surge voltage characteristics, though the value may change depending upon the experimental condition. However, this chopping current value results in an allowable surge voltage for loads having high dielectric strength such as a rotary machine (motors) and a transformer but it is yet too high for loads having low dielectric strength such as a dry type transformer and is likely to cause dielectric breakdown. In designing various reception and distribution equipments, it has been difficult to attain completely surge-absorberless vacuum circuit breakers by use of the conventional low surge voltage vacuum circuit breakers. In other words, the low surge voltage vacuum circuit breakers have been made most of only in the limited application or only for a load system having high dielectric strength.
Among the factors that determine the low surge voltage characteristics, the chopping current characteristics inherent to the electrode material are the most dominant. Generally, the above-mentioned switching surge voltage V can be expressed as V.apprxeq.P.multidot.Is.multidot.Z where Is is a chopping current value of an electrode material, Z is a surge impedance of a load machine and P is the damping constant dependent on the load, and .apprxeq. means "nearly equal to". In order to prevent dielectric breakdown of the dry type transformer or the like in the light of this relation, it is necessary to reduce the chopping current value Is to the order of 1A and preferably, below 1A. Accordingly, the conventional material is not satisfactory for the surge-absorberless vacuum circuit breaker.